For cutting, die cutting and perforating sheet materials such as non-wovens, paper, foils, films and woven materials, such sheet materials generally are cut manually or, in more automated processes, using sheet length cutting devices such as a rotary cutter. In conventional manual cutting systems, the sheet material generally is run to a desired length and is stopped by either stopping the machine or through the use of an accumulator. After the machine has been stopped, or a sufficient amount of material received in the accumulator, a knife or cutter, such as a guillotine or rotary shears, is engaged to cut the desired length of sheet material. Such a method is, however, somewhat time consuming as the material must be continually stopped and started prior to and after a cut is made. Further, if the converting machine from which the material is fed is operating at speeds that are too high, i.e., is passing the sheet material at too fast a rate for the use of an accumulator, or if the machine cannot be readily and easily stopped and started, a rough, jagged cut often results making the use of such stationary or manual cutters impractical.
Rotary cutters generally are designed to make cuts in a moving web or sheet material such as a non-wovens, paper, film, foil, or a woven material that is moving at high speeds without stopping to form the desired lengths of sheet material. Such rotary cutters generally include a rotating anvil roll made from a hardened steel or similar material, and a rotating knife roll having a knife blade or blades mounted thereon. Generally, the circumference of the knife roll dictates the length of sheet material to be cut. In order to achieve a clean cut in the web or sheet material, the rotary knife generally must be moving at or close to the speed of the material being cut and must be matched with the speed of rotation of the anvil roll. When the knife engages the material at the same speed as the material is moving, a clean cut is made just as if the material were stopped and the cut made by scissors or stationary knife.
Rotary cutters are, however, generally limited to a set cut length as the circumference of the knife cylinder generally must match the length of the cut desired. If the sheeting line is not dedicated to cutting only one cut length and instead it is necessary or desired to cut different lengths of sheet materials on the same sheeting line, it becomes necessary either to change the knife rolls to substitute different knife rolls having circumferences matched to the different cut lengths desired, or to vary the speed of the rotation of the knife rolls to shorten or lengthen the cut made by the knife roll. The primary problem with changing out the knife rolls is the expense and labor required to make such a change in the knife roll. The knife rolls for such rotary cutters are extremely expensive, typically costing tens of thousands of dollars, and given the size and weight of these rolls, their change out is somewhat labor intensive. In addition, since the cut length permitted by the knife rolls is limited to the circumference of the knife rolls, each knife roll still is able to accommodate only one cut length. Therefore, given the high costs and labor required for changing such rotary cutters, maintaining an inventory of several different sized knife rolls for cutting varying lengths of sheet material generally is impractical, especially given the number of variations in cut lengths that could be required.
Attempts further have been made to try to vary the length of the cut made by the knife roll of a rotary cutter by varying the speed of the knife roll with respect to the speed of movement of the sheet material. However, such variations in the speed of rotation of the knife roller typically result in ragged or rough cuts due to bunching or wrinkling of the material behind the knife where the knife roll is moved at a slower rate than the rate of movement of the material, or as the knife is moved at a faster rate than the rate of movement of the material, the material tends to be pulled and thus torn or ripped. This is especially a problem for cuts greater than the circumference of the knife roll. Clutch drive systems have been developed to try to solve this problem of controlling the rotational speed of the knife roll for varying the length of cut of the sheet material. In such system, clutches are engaged and disengaged to speed up and slow down the knife roll. The problem with such clutches is they provide inaccurate control of the rotation of the knife roll as clutches tends to slip and do not always tends to engage at the same rate. Thus, conventional clutch systems generally have not provided desired reliability and control to ensure a clean and accurate cut at a desired length.
In addition, there have now been developed computer controller servomotor driven rotary cutting systems that use servomotors or hydraulic drives programmed to control the speed of rotation of the knife roll. Such systems are designed to vary the speed of the knife roll during a cycle of rotation to shorten or lengthen the cut length of the sheet material as desired. Such electronic or computer controlled servomotor systems, are, however, limited to a narrow range of cut lengths, and the size and weight of the knife rolls and anvil rolls generally requires larger, more powerful servomotors for greater ranges of speed during a cycle of rotation. In addition, such systems are extremely expensive over and above the expense of the knife rolls themselves, in many cases more than doubling the cost of the rotary cutting systems.
Accordingly, it can be seen that a need exists for a variable length rotary cutting system that can be quickly and easily adjusted to enable variations in the cut length provided by the rotary cutter without requiring expensive servomotors or controls, and which can achieve a clean, accurate cut of desired length of sheet material being run in a moving web at high speeds, without stopping.